We have undertaken to characterize the cardiac cell surface molecules responsible for regulating ionic conductances in the heart, with particular emphasis on developmental aspects of automaticity ("pacemaker" activity). The reversion to more primitive electrophysiologic behavior (tettrodotoxin-insensitive "slow" sodium channels and spontaneous diastolic depolarization) classically observed with embryonic chick cardiac myocytes grown in monolaryer culture in the presence of fetal bovine serum, was prevented in serum-free, hormally-defined medium that had been supplemented with insulin, dexamethasone, and fetuin. Further work is necessary to establish the biochemical basis and precise ionic mechanisms underlyuing the succession of electrophysiologic phenotypes during differentiation. a reporated high-affinitay antagonist of the calkcium-dependent potassium channel, the putative "pacemaker", is apamin, a polypeptide neurotoxin from bee venon. Apamin purified by reverse-phase HPLC will be used as a molecular probe for "pacemaker" development, solubiliztion and affinity chromatography. The specific eluate will be used to produce monoclonal antibodies directed against various channel antigenic domains to elucidate "pacemaker" differentiation and function.